People in today's world rely heavily on a wide variety of electrical devices. Almost all of these devices draw power ultimately from a commercial source, usually delivered to the user through a wall outlet or socket. Most wall sockets have two (duplex) or four (quad) adjacent plugs.
A large number of these devices require a low voltage DC power input, typically available through a power supply. The required power supplies tend to be large, even those for providing small current outputs. Unfortunately, most small power supplies are too bulky to be plugged into a wall socket and still allow room for other devices, even simple plugs but also including other power supplies, to be plugged into the same wall socket.
A wide variety of modem devices need only a low-output power supply, typically up to 20 W with output voltages of 3.3, 5, 12, 21 and 24 VDC. Modem telephones, answering machines, radios, recording machines, facsimile machines, computer accessories such as modems, and many others are designed without internal power supplies and instead rely on an external power supply. Another large class of electrical devices is various portable electrical devices that use rechargeable batteries. For many devices, such as portable phones, electronic cameras, video games, calculators, tape players, and the like, a battery is or can be fitted within the device itself. For some devices, the battery can be removed easily, while in other devices, the battery is not designed to be removed.
A variety of power supplies and battery chargers are available for use with such devices. In general, a manufacturer provides a power supply or battery charger specifically designed for use with one or more products. Traditionally, the power supplies or battery chargers are bulky devices, often weighing 500 grams or more. The specific size of the battery charger is determined by a number of factors, including power to be delivered during product use or recharging and the presence or absence of special circuitry, for example to monitor the state of charge of a battery.
In one typical configuration, a power cord goes between the power supply and a wall socket, with a second power cord extending between the power supply and the electrical device. In another typical configuration, the power supply is built into a module which is designed to be plugged directly into a wall socket, with a single cord connecting the power supply to the battery. Traditional wall-mounted power supply modules have been relatively large. However, even the new, smaller wall-mounted power supplies provide only limited power or are too large for many applications.
Many power supplies or battery chargers are designed to be wall-mounted simply by virtue of plugging into a wall socket. A typical device includes a casing which terminates in a plug which is designed to plug directly into the wall socket. The casing is often designed to lie against a wall to provide mechanical stability and to maintain the plug prongs in proper contact with the wall socket.
Current wall adapters suffer from some significant problems. The most important is that the adapters are bulky and cover more than a single wall socket. In addition, the weight of current adapters can be a problem, especially when several adapters are plugged into a multi-outlet power strip.
Unfortunately, electrical plugs have an orientation and various power supplies are designed to extend in different directions relative to the plug. For example, a traditional American plug has two, parallel flat prongs, with the neutral prong slightly wider than the hot prong, plus, for many plugs, a cylindrical ground lead positioned relative to the flat prongs to form a triangle. Wall sockets are usually installed with the ground socket below the prong sockets, but this orientation is sometimes altered. Most power supplies are designed so the bulk of the device extends away from the plane of the two prongs in the direction of the ground lead, but others have the opposite orientation and a few are rotated by 90.degree..
The problem is accentuated in that most power supplies have a power cord running from the power supply for some distance to an adapter plug or an electrical device and this cord is normally positioned perpendicular to and pointing away from the plane defined by the two main prongs. This extends still farther the area covered by the power supply.
Few, if any, power supplies position the bulk of the device parallel to the prongs (extending away from the wall in a typical installation) because this would result in a large mass on a long lever arm. Since most plug devices are designed to be secured by spring tension and interaction with a wall plug, this can pose a significant mechanical disadvantage. The increased lever arm will tend to shift the plug downward, tending to pry the plug out of the wall socket. Angling the plug severely can compromise the electrical connection to the point that the plug no longer is in electrical contact with source current. This type of angling may lead to partial separation from the wall socket and may expose the prongs of the plug in such a way that a person, or even an animal, might come into contact with live current, thereby causing bodily harm.
These problems arise because previously available power supplies simply could not be built to provide enough power and also to fit within a small package, for instance in the general shape of a conventional plug.
The specifics of internal electronics have been less of a problem, at least in terms of providing necessary electronic functions. There is no world standard for power supply voltage or frequency, but since many electronic devices, and essentially all battery powered electrical devices, ultimately run on direct current, it is not too difficult to design a "universal" power supply that converts 100-240 volts AC at 50-60 Hertz into a direct current suitable for a particular application. Thus, industry has been able to address the power component and provide universal solutions. The size and bulk of power supplies, however, remain a problem.
Typical existing wall adapters utilize some form of electronic voltage regulation. The general techniques are well known in the art. The cheapest form of regulation is linear, typically using a series regulator or shunt regulator. The bulk, weight and substantial heat dissipation requirements of linear regulators are well known. For certain applications, a switching power supply may be useful, but such supplies require careful engineering to avoid conducted and radiated RFI. In addition, switching power supplies are prone to subtle failure modes that may be difficult to predict or analyze.
Linear regulated power supplies have distinct drawbacks over the more sophisticated switching power supplies. Firstly, a linear power supply requires a 50-60 Hz transformer to step down the AC voltage. The size of the transformer increases exponentially as the output power requirement increases above 5 W. Secondly, linear regulators are limited to a single output voltage and as a result, an entirely separate regulator must be added for each additional output voltage. Since multiple output voltages are important for many devices, this severely limits the usefulness of a linear regulator. For example, a typical device may require 12 V for a disk drive plus 3.3 or 5 V for logic circuits (sometimes both). Additionally, linear power supplies operate at only 30-60% efficiency, making it necessary to have a large linear transformer plus a large heatsink to the pass transistor. These shortcomings increase both the cost and size of the power supply.
Switching mode power supplies yield operating efficiencies in the 68-90% range. (Switching power supplies which exceed 80% efficiency are typically DC-DC units. In common practice, AC-DC models operate in the 68-75% efficiency range.) The power transistors used in switching power supplies deliver many times their power rating to the load and the circuit allows for multiple outputs from a single transformer. In addition, the operating frequency is much greater than 60 Hz, allowing designers to use many smaller components. All of this makes a switching power supply smaller and less costly than a linear regulated model with comparable output, particularly for power levels above about 5 W. However, even the best switching power supplies available before the present invention were too low current or too large to be as compact as the present device and thus suffer from the size problems discussed above.
The new device of this invention overcomes these problems by providing a compact, light weight, high output power supply that can be completely incorporated in a slightly oversize plug body.